I thought this was a general 3.6.1 issue as it only started after upgrading, but I have a number of WordPress blogs and this is only happening with the Manifest theme.
When I open a draft post to edit, make some editions, then click "Save Draft" it does so without a problem - but only the first time. Thereafter, if I continue to edit and click "Save Draft" for a second time, instead of simply saving the edit, it opens up a Preview of the post in a new window (just like pressing 'Preview). It DOES save the post, but when i go back to the edit page, the 'Save Draft' button is greyed out with a spinning wheel of death next to it, and it stays that way until I leave the edit page.
In the meantime, the button does seem to save a post, although each time, it still opens a Preview page as above.
I tried this on three other blogs and could not replicate the issue.
The blog I need help with is redpathhouse.wordpress.com.